The present invention relates to steam turbines and, more particularly, to a method and apparatus for reducing erosion-corrosion in steam turbine exhaust systems.
Water droplets in steam turbine exhaust systems are known to produce erosion-corrosion. Such erosion arises from the effects of the water droplets impacting and flowing on surfaces within the exhaust system and has been termed flow assisted corrosion. The effect is particularly noticeable in exhaust pipes and is severe beyond bends in such exhaust pipes. Droplet formation at bends is created by turbulence induced in the steam flow as it passes around such bends. This droplet formation phenomena is well known and various efforts have been made to reduce the amount of droplet formation which occurs at such bends. One typical solution to the problem of droplet formation is to incorporate turning vane assemblies in the bends to separate the steam flow into a plurality of channels and to reduce the strength of vortices thus limiting the turbulence and minimizing the creation of such droplets. The turning vane assemblies essentially reduce the development of undesirable flow characteristics in a high velocity fluid stream. However, the turning vanes create an additional problem in that water droplets in the steam tend to collect on the vanes and along the walls of the bend and then re-enter the flow as considerably larger and therefore more erosive droplets.
Various proposals have been submitted in an effort to alleviate the erosion-corrosion problem created by water droplets in steam. In general, it is known to incorporate water collection devices into steam exhaust pipes to attempt to attract the water droplets and remove them from the steam flow. A disadvantage of water collection devices associated with the high velocity exhaust steam flow is that such devices must not adversely affect the flow characteristics and must be physically capable of collecting most of the water in their immediate vicinity. The device must also be positioned at a point that provides access to as much of the water as possible. At least some water collectors have been positioned on the suction side or downstream side of the turning vanes themselves in order to attempt to collect the water which forms on the turning vanes. However, such positioning on the turning vanes does not always resolve the problem of positioning the collector at a point where the water tends to accumulate. Additionally, the collectors physical capability of collecting the water is adversely affected by pressure differentials in the vicinity of the collector. The pressure differentials across the surface of a turning vane may vary significantly from the leading edge to a trailing edge. In some designs it has been noted that a portion of the collecting surface may be at a higher pressure than another portion of the collecting surface resulting in water entering the high pressure part and being expelled out of the low pressure part without being collected.